The problem "What is it" has always changed its contents due to the history of philosophy. In this article, the author intends to make clear what happened in the problem of "What is it" by giving an overlook of the philosophical development from the early Greek, the Middle ages down to the modern time. This article is especially concerned with the problem how to understand the "Being" connected with the ideas of Time and Space. The problem of Chaos is mentioned the end of this paper, waiting for further studies.